In the manufacture of coated translucent materials, such as substrates provided, e.g., with electrically conductive coatings as used in photovoltaics, the inspection of the transparent and reflective properties of the substrate is important mainly with regard to achieving an optimum efficiency in the conversion of solar radiation to electricity.
An important parameter in this is haze, a wide-angle scattering of light. According to the American Society for Testing and Materials international ASTM D standard 1003 the detailed specification of which is hereby incorporated by reference herein, haze is defined as the percentage of the light passing a translucent object that is deflected in the object by more than 2.5° from its propagation direction and thus, when passing the object, is scattered out of a directed bundle of rays due to, e.g., surface roughness of the object.
Pursuant to ASTM D 1003, the haze of translucent objects (transmission haze) can be determined by placing the object in front of the light entrance port of an integrating sphere and illuminating it from the object side opposite the integrating sphere in such a way that the illuminating light transilluminates the object and enters the integrating sphere. The integrating sphere features a built-in photodetector, which receives the transmitted light and converts it into a measurement signal. The detection direction of this photosensor and the normal to the object surface (which normal constitutes a measuring axis) include a specified angle.
In a different measuring method also specified in ASTM D 1003, the integrating sphere itself is used as an illumination device. By means of the integrating sphere, the object placed in front of the sphere's light exit port is illuminated with diffuse light. In this case, the photodetector is arranged on the other side of the object, i.e., the side opposite the integrating sphere.
Within the integrating sphere in the latter case, a light trap is provided which can be activated and which, in the active state, blocks out the directional portion of the light so that it does not reach the photodetector. By alternately activating and deactivating the light trap, one can determine two different transmission parameters. One of them delivers a measure of diffuse transmittance (Tdiffuse), i.e., the light scattered at an angle of >2.5° due to the influence of the object, whereas the other parameter delivers a measure of total transmittance (Ttotal), with light scattered by the object at an angle of ≦2.5° only, if at all. From these two data, the haze of the object is determined by the formula Haze=Tdiffuse/Ttotal·100%.
German National Patent DE 100 10 213 B4 captioned, “Optical Reflection and Transmission Measurement Device for Quality Monitoring in a Continuous Process, has Signal Conditioning Unit Integrated in Measuring Head to Process Output Signals of Spectrometer,”, which operates by the principle of spectroscopy is hereby incorporated herein. A pair of corresponding U.S. patent applications both captioned, “Optical Measuring Arrangement, in Particular for Quality Control in Continuous Processes” and published as US2003202180 (A1) and US2002001078 (A1) are also fully incorporated by reference herein. The measuring apparatus depicted and described in the foregoing patent records features a measuring head with an integrating sphere, which is used for the diffuse illumination of the measuring object. With this arrangement, the total transmittance (Ttotal) of the illuminating light through the object and the reflectance of the illuminating light from a surface of the object can be measured sequentially.
German National Patent DE 10 2009 040 642 B3 describes a method for measuring optical parameters of transparent, scattering measuring objects and an apparatus for applying this method, especially for the in-line measurement of various transmittance and reflectance data of plate-shaped or strip-like substrates that are given transparent coatings in a coating plant and it too is hereby incorporated by reference herein.
With the latter method, too, the measuring object is illuminated with diffuse light by means of an integrating sphere. The light passing the measuring object is simultaneously captured by means of two photodetectors having differing detection directions; in one of the two detection directions the radiation directed immediately at the photodetector is suppressed by means of a light trap. By this method and the apparatus for its execution it is possible to simultaneously determine the diffuse transmittance (Tdiffuse) and the total transmittance (Ttotal). Provided in addition are two more photodetectors, of which one detects the diffuse luminous reflectance, and the other, the total luminous reflectance from one of the surfaces.
In connection with process monitoring and quality inspection in the manufacture of large-surface coated substrates, it is frequently required to determine both diffuse transmittance (Tdiffuse) and total transmittance (Ttotal) as well as luminous reflectance from both surfaces of the substrate, i.e., reflectance from the coated as well as from the uncoated substrate surface, and that with low technical expenditure. With the methods and apparatuses available in prior art, this requirement cannot be met.
Therefore, the disclosure is based on the problem of finding a method and at least one apparatus that do not suffer from the drawbacks of prior art as described above.